In recent years, various techniques for crystallizing or improving the crystallinity of an amorphous or polycrystalline semiconductor film have been investigated. This technology is used in the manufacture of a variety of devices, such as image sensors and active-matrix liquid crystal display (AM-LCD) devices. A regular array of thin-film transistors (TFTs) is fabricated on an appropriate substrate and each transistor serves as a pixel controller.
Crystalline semiconductor films, such as silicon films, provide pixels for liquid crystal displays. Such films have been processed by irradiation by an excimer laser followed by crystallization in excimer laser annealing (“ELA”) processes. Other more advantageous methods and systems, such as sequential lateral solidification (“SLS”) techniques, for processing semiconductor thin films for use in liquid crystal displays and organic light emitting diode (“OLED”) displays have been described. SLS is a pulsed-laser crystallization process that can produce crystalline films on substrates, including substrates that are intolerant to heat such as glass and plastics.
SLS uses controlled laser pulses to melt a region of an amorphous or polycrystalline thin film on a substrate. The melted region of film then laterally crystallizes into a directionally solidified lateral columnar microstructure or a location-controlled large single crystal region. Generally the melt/crystallization process is sequentially repeated over the surface of a large thin film large, with a large number of laser pulses. The processed film on substrate is then used to produce either one display, or even divided to produce multiple displays.
However, conventional ELA and SLS techniques are limited by variation in the laser pulses from one shot to the next. Each laser pulse used to melt a region of film typically has a different energy fluence than other laser pulses used to melt other regions of film. In turn, this can cause slightly different performance in the regions of recrystallized film across the area of the display. For example, during the sequential irradiation of neighboring regions of the thin film, a first region is irradiated by a first laser pulse having a first energy fluence; a second region is irradiated by a second laser pulse having a second fluence which is at least slightly different from that of the first laser pulse; and a third region is irradiated by a third laser pulse having a third fluence that is at least slightly different from that of the first and second laser pulses. The resulting energy densities of the irradiated and crystallized first, second and third regions of the semiconductor film are all, at least to some extent, different from one another due to the varying fluences of the sequential beam pulses irradiating neighboring regions
Variations in the fluence and/or energy density of the laser pulses, which melt regions of film, can cause variations in the quality and performance of the crystallized regions. When thin film transistor (“TFT”) devices are subsequently fabricated in such areas that have been irradiated and crystallized by laser beam pulses of different energy fluences and/or energy densities, performance differences may be detected. This may manifest itself in that the same colors provided on neighboring pixels of the display may appear different from one another. Another consequence of non-uniform irradiation of neighboring regions of the thin film is that a transition between pixels in one of these regions to pixels in the next consecutive region may be visible in the display produced from the film. This is due to the energy densities being different from one another in the two neighboring regions so that the transition between the regions at their borders has a contrast from one to the other.